U.S. Pat. No. 5,071 624 discloses an arrangement for the enrichment of sample substances for spectroscopical purposes in a flow-injection analysis, wherein an ion exchange column is used for the enrichment. In this arrangement the flow-injection means serve to apply the sample substance to the ion exchange column, to elute it therefrom in enriched form and to supply it to sample feeding means in the form of a dosing capillary tube.
From the dosing capillary tube, the enriched sample is dispensed into a tubular furnace of an atomic absorption spectrometer.
In this arrangement, the flow-injection can only be used for the supply of the sample substance. Sample treatment, particularly sample disintegration, is not provided.
Furthermore, numerous arrangements for use in flow-through analysis are known. Thus, U.S. Pat. No. 5,045,196 shows an ion exchange column. German patent document 3,833,248 shows a device for supplying a well-defined sample volume to an instrument. A paper by Jaramier Rudzicker, "Homogeneous and Heterogeneous Systems Flow Injection Analysis Today and Tomorrow", in "Analytica Chimica Acta", 214 (1988), pp. 1-27 shows a pump arrangement for admixing. German patent 3,606,938 shows a diaphragm type admixing device. German patent 2,842,864 shows a device for flow extraction, wherein one substance is solved in a first solvent phase and is extracted with a second solvent phase, which is unmixable with the first solvent phase. German utility model 90 13 193 shows the modular construction of a flow-injection analysis system.
British patent document 2,099,579 discloses the use, in flow-injection systems of components which consist of polytetrafluorethylene. A paper "Die FlieBinjektionsanalyse (FIA) in der Atomspektrometrie" in "Nachrichten chemische Technik LAB" 39 Nr. 3 (1991), pp. 310-315 discloses the use of atomic spectrometers, atomic absorption spectrometers and flame atomic absorption spectrometers as analytical instruments in flowinjection analysis. From European patent document 0,212,241, it is known to direct volatile hydrides optionally to a plasma emission instrument or to a heated measuring cuvette for the atomic absorption spectroscopy. In this respect, furthermore, the paper by Masashi Goto et al. "Continuous micro flow monitoring method for total mercury at sub-ppb level in wastewater and other waters using cold vapor atomic absorption spectrometry" in Fresenius Z Anal. Chem. Jg. 332(1988) pp. 745-749 is to be mentioned. The use of atomic absorption spectrometers as analytical instruments in flow-injection systems is also known from a paper by Backstrom, Kenneth and Danielsson, Lars-Goran: "Sample Work-up for Graphite Furnace Atomic-absorption Spectrometry" Vol. 109, March 1984, pp. 323-325. A paper by Mertens, H and Althaus, A "Bestimmung von Quecksilber mit Hilfe der Amalgamtechnik unter Verwendung von Hydroxylammoniumchlorid und Natriumborhydrid oder Zinn(II)-chlorid" in "Fresenius Z Anal. Chem." Issue 7, 983, pp. 696-698 discloses the use of aerosol separators in the inert gas flow and the use of mercury adsorbers. Disclosure in this respect is also contained in German patent document 3,917,956, German patent document 3,503,315 and U.S. Pat. No. 4,816,226.
It is also known to use microwave ovens in analytical techniques for the disintegration of a sample. Thus, U.S. Pat. No. 4,946,797 describes the use of a microwave oven for wet disintegration in the determination of nitrogen according to Kjeldahl. From JP 1-308940 A in "Patent Abstracts of Japan" P-1014, Feb. 28, 1990, Vol 14/no 11, it is known to use a microwave oven in gas or liquid chromatography for treating a mixture, which has previously been treated with ultrasonics.
From the paper by M. Burgurea and J. L. Burgurea in Analytica Chimica Acta 179 (1986), pages 351 to 357 under the title "Flow-injection and microwave-oven sample decomposition for determination of copper, zinc and iron in whole blood by atomic absorption spectroscopy" it is known to use a microwave oven in connection with flow-injection in order to disintegrate a sample for the subsequent measurement of the atomic absorption. To this end, the flow-injection system comprises a double valve by means of which a blood sample and a disintegrating agent, namely a mixture of hydrochloric acid and nitric acid, are injected into separate carrier liquid flows which are combined at the inlet of a flow-through reactor. The microwave oven is a normal household appliance, and the flow-through reactor forms a tubular conduit of pyrex which is wound to a coil and arranged in the radiation cavity of the microwave oven. After having passed through the flow-through reactor, the disintegrated sample is introduced into the nebulizer of a flame atomic absorption spectrometer.
In this publication it is explained that the volumes of the sample and of the disintegrating agent as well as the length of the pyrex tube and the flow rate have to be adjusted such that the dwell time in the microwave oven, on one hand, is sufficent for the complete disintegration of the sample and, on the other hand, is not so long that steam or gas bubble formation in the liquid is caused in the flow-through reactor. Such steam or gas bubble formation has a negative effect on the flow-injection analysis because it results in nonreproducible dispersion, i.e. means mixture of the sample with the carrier liquid.
In a paper by S. Hinkamp and G. Schwedt in Analytica Chimica Acta 236 (1990), pages 345 to 350, under the title "Determination of total phosphorus in waters with amperometric detection of total coupling of flow-injection analysis with continuous microwave oven digestion", a microwave oven is described in connection with a flow-through reactor. The flow-through reactor consists of a tubular conduit of polytetrafluorethylene wound to a coil, on the inlet side of which a carrier liquid flow with water samples containing phosphate is combined with a flow of disintegrating agent. Here, the disintegrating agent consists of a solution of peroxodisulphate or perchloric acid, through which all of the phosphorous compounds are transformed into the orthophosphate at the pH of the carrier liquid The microwave oven which likewise is a household appliance, further includes a water cooler.
After the transformation in the flow-through reactor, the carrier liquid flow passes through a gas diffusion cell in which gas bubbles are removed from the liquid flow. Subsequently, the carrier liquid flow with the disintegrated sample is combined with a reagent flow (acid ammonium molybdate solution) in a further flow-through reactor and subsequently gets into an electrochemical gauge head in which the formed molybdophosphate is reduced amperometrically to molybdenum blue and, thus, the contents of phosphate is determined in the sample.
Microwave ovens of the type used as household appliances have a power of 650 Watt and, thus, they are considerably overdimensioned with regard to the relatively small liquid quantities which have to be heated in flow-injection analysis. As can be seen from the illustration in FIG. 1, very low microwave powers are sufficient to heat the liquid in the microwave oven to the desired disintegration temperature. FIG. 1 shows the achievable liquid temperature in the form of the temperature difference .DELTA.T in .degree. C. relative to ambient temperature as a function of the flow rate and the irradiated microwave power. It can be seen that only small fractions of the total power of 650 Watt are required in order to achieve the desired disintegration temperature. At the same time, however, also the problem arises that disintegration reactions also at higher temperatures need a certain minimum of time in order to definitely complete the desired disintegration, because otherwise the analysis result is falsified. These problems are additionally complicated in that the required disintegration temperatures and disintegration times depend on the matrix which is contained in the element to be determined. Thus, all in all, a high variability in the adjustment of the disintegration conditions is required, because the disintegration conditions vary from sample to sample. This variability is not present in the known devices, because, due to the high power of the microwave ovens, certain conditions with regard to conduit diameter and flow rate have to be maintained in order to avoid overheating and the vapour and gas bubble formation resulting therefrom, which would cause an unreproducible dispersion. Then, also additional means for removing the vapour and gas bubbles would have to be provided.